1. Field of the Invention
The present invention relates to a transfer unit of an electrophotographic printer and, more particularly, to a transfer unit of an electrophotographic printer having an improved structure for establishing contact or separation between a transfer roller and a fixing roller between which a paper passes through for printing an image on the paper.
2. Description of the Related Art
As shown in FIG. 1, an electrophotographic printer such as a color laser printer includes a photosensitive belt 10 traveling along a circular path, a developing unit 20 for developing a desired image on the photosensitive belt 10, and a transfer unit 30 for printing the image developed on the photosensitive belt 10 by the developing unit 20.
The transfer unit 30 includes a transfer roller 31 to which the image developed on the photosensitive belt is transferred and which prints the transferred image on one surface of a paper 1, a fixing roller 32 which presses the paper 1 against the transfer roller 31 while rotating in contact with the transfer roller 31, first and second levers 34 and 35 which support the transfer roller 31 and the fixing roller 32, respectively, and pivot around a hinge shaft 33a, and pivoting means for pivoting the first and second levers 34 and 35.
The pivoting means includes a first screw 38 which passes through the first lever 34 and is fixed to a base frame 33, a first compression spring 38a which is installed around the circumferential surface of the first screw 38 and presses the first lever 34 downward, a second screw 39 which passes through the second lever 35 and is fixed to the first lever 34, a second compression spring 39a which is installed around the circumferential surface of the second screw 39 and presses the second lever 35 downward, and first and second cam members 36 and 37 which support the first and second levers 34 and 35, respectively upward.
A torque around the hinge shaft 33a in a counterclockwise direction in FIG. 1 acts on the first and second levers 34 and 35 by the elastic forces of the first and second compression springs 38a and 39a. Therefore, the photosensitive belt 10, the transfer roller 31, and the fixing roller 32 can be moved to be in contact with each other. On the other hand, since the first and second cam members 36 and 37 each rotate, a predetermined amount, the photosensitive belt 10, the transfer roller 31, and the fixing roller 32 can contact each other or separate from each other. That is, when the first and second cam members 36 and 37 rotate in a clockwise direction, the first and second levers 34 and 35 are raised while overcoming the elastic forces of the first and second compression springs 38a and 39a, and the photosensitive belt 10, the transfer roller 31, and the fixing roller 32 separate from each other. In addition, when the first and second cam members 36 and 37 rotate in a counterclockwise direction, the first and second levers 34 and 35 are lowered by the elastic forces of the first and second compression springs 38a and 39a, and the photosensitive belt 10, the transfer roller 31, and the fixing roller 32 contact each other.
However, in the transfer unit configured as above, a relatively large driving force is required to pivot the first and second levers 34 and 35. For example, when the individual pressing forces between the photosensitive belt 10 and the transfer roller 31, and between the transfer roller 31 and the fixing roller 32 are 80 kgf, the first and second compression springs 38a and 39a each having an elastic force of 40 kgf are installed in pairs on the side shown in FIG. 1 and on an opposite side hidden by the shown springs 38a and 39a. On the other hand, while only the elastic force of the second compression springs 39a, i.e., 80 kgf acts on the second cam member 37, the summed elastic force of the first and second springs, i.e., 160 kgf acts on the first cam member 36. Therefore, there is a problem in which a greater force is required to cause the photosensitive belt 10, the transfer roller 31, and the fixing roller 32 to contact or separated from each other, and thus power consumption is high.